FIG. 7 is an explanatory view showing the state of a two-axle truck 1 for a railway car when traveling along a curved track.
As shown in FIG. 7, when the truck 1 is traveling along a curved track, the force F1 to the left and right (referred to in this description as the “lateral force”) between the wheel 3 on the outer side of the wheelset 2f on the front side in the direction of travel (referred to below as the front wheelset) and the rail 5 on the outer side of the curved track increases.
FIG. 8 is an explanatory view showing the relationship between the wheels 3 of the front wheelset 2f when traveling along a curved track and the rails 5 of the curved track.
As shown in FIG. 8, when a truck is traveling along a curved track, the attack angle α which is the relative yawing angle between the front wheelset 2f and the rails 5 (as shown in FIG. 8, on a curved track, the direction in which the wheelset faces the outer rail is given a positive value) increases. Therefore, excessive sliding of the wheels 3 in the lateral direction (referred to as lateral creep) develops. As a result, as shown by the arrow in FIG. 7, yawing develops in the truck 1, and a lateral force F1 develops in the front wheelset 2f of the truck 1.
On the other hand, in the wheelset 2r on the rear side in the direction of travel (referred to below as the rear wheelset), there is almost no lateral displacement, and there is almost no difference in the radius of rotation of the left and right wheels 3. Therefore, the necessary difference in the wheel radius between the inner and outer rails 5 (referred to as the pure rolling radius difference) cannot be obtained, and excessive sliding of the wheels 3 in the longitudinal direction (referred to as longitudinal creep) develops. The longitudinal creep force F2 in the rear wheelset 2r which develops in this manner causes an increase in the lateral force F1 in the front wheelset 2f. 
Non-Patent Document 1 listed below describes that there is a large possibility that firstly longitudinal creep in the rear wheelset and secondly lateral creep in the front wheelset are the primary causes of rail corrugation which develops in the inside rail of a sharp curved track. Thus, it is thought that the occurrence of this rail corrugation can be suppressed if lateral creep (lateral pressure) of the front wheelset and longitudinal creep of the rear wheelset can be decreased.
Patent Document 1 listed below discloses, with the intension of decreasing the lateral pressure, an invention which varies the rigidity of the front and rear axle box suspensions which rotatably support the front wheelset and the rear wheelset, respectively, and an invention in which the wheels of the rear wheelset are independent of each other.    Patent Document 1: Japanese Patent No. 2738114    Non-Patent Document 1: “Properties of Trucks and Tracks When Traveling Along a Sharp Curve and Their Effect on Rail Corrugation”, J-Rail 1995 (1995 Railway Technology Joint Symposium in Japan)